A conventional commercial solid electrolytic capacitor generally consists of a porous metal electrode, an oxide layer located on the metal surface, an electrically conductive solid which is introduced into the porous structure, an outer electrode (contacting), such as for example a silver layer or a cathode foil, and also further electrical contacts and an encapsulation.
Examples of solid electrolytic capacitors are tantalum, aluminium, niobium and niobium oxide capacitors with charge transfer complexes, brownstone or polymer solid electrolytes. The use of porous bodies has the advantage that it is possible to achieve, owing to the large surface area, a very high capacitance density, i.e. a high electrical capacitance in a small space.
The high electrical conductivity of π-conjugated polymers makes them particularly suitable as solid electrolytes. π-Conjugated polymers are also referred to as conductive polymers or as synthetic metals. They are becoming increasingly economically important, as polymers have advantages over metals with respect to processability, weight and the targeted setting of properties by chemical modification. Examples of known π-conjugated polymers are polypyrroles, polythiophenes, polyanilines, polyacetylenes, polyphenylenes and poly(p-phenylenevinylenes), poly-3,4-(ethylene-1,2-dioxy)thiophene, often also referred to as poly(3,4-ethylenedioxy-thiophene), being a particularly important and industrially used polythiophene, as it has in its oxidised form very high conductivity and high thermal stability.
Technical developments in electronics are increasingly calling for solid electrolytic capacitors with very low Equivalent Series Resistances (ESRs). The reason for this is for example falling logic voltages, higher integration density and rising clock frequencies in integrated circuits. Furthermore, a low ESR also reduces the amount of energy consumed; this is particularly advantageous for mobile, battery-operated applications. It is therefore desirable to reduce the ESR of solid electrolytic capacitors as much as possible.
European patent specification EP-A-340 512 describes the production of a solid electrolyte from 3,4-ethylene-1,2-dioxythiophene and the use of its catatonic polymer produced by oxidative polymerisation as a solid electrolyte in electrolytic capacitors. Poly(3,4-ethylenedioxythiophene), as a substitute for manganese dioxide or for charge transfer complexes in solid electrolytic capacitors, reduces, on account of the higher electrical conductivity, the equivalent series resistance of the capacitor and improves the frequency response.
In addition to a low ESR, modern solid electrolytic capacitors also demand a low residual current and good stability in relation to external loads. In particular during the production process, high mechanical loads, which can greatly increase the residual current of the capacitor anode, occur during the encapsulation of the capacitor anodes.
Stability in relation to loads of this type, and thus a low residual current, may be achieved above all by an approx. 5-50 μm-thick outer layer made of conductive polymers on the capacitor anode. A layer of this type serves as a mechanical buffer between the capacitor anode and the cathode-side contacting. This prevents the silver layer (contacting), for example, from entering during mechanical loading into direct contact with or damaging the dielectric and the residual current of the capacitor from increasing as a result. The conductive polymeric outer layer itself should display what is known as self-healing behaviour: Relatively minor defects in the dielectric at the outer anode surface, which occur despite the buffer effect, are electrically isolated as a result of the fact that the conductivity of the outer layer is destroyed at the site of the defect by the electrical current.
Forming a thick polymeric outer layer by means of in-situ polymerisation is very difficult. The layer formation requires in this case very many coating cycles. The large number of coating cycles makes the outer layer very inhomogeneous; the rims of the capacitor anode, in particular, are often inadequately covered. Japanese patent application JP-A 2003-188052 describes that homogeneous rim coverage requires costly adaptation of the process parameters. However, this makes the production processes very susceptible to malfunctions. In addition, it is generally necessary to remove residual salts from the layer, which is polymerised in situ, by washing; as a result, holes faun in the polymer layer.
A dense electrically conductive outer layer with good rim coverage can be achieved by electrochemical polymerisation. However, electrochemical polymerisation requires a conductive film first to be deposited on the insulating oxide layer of the capacitor anode and this layer then to be electrically contacted for each individual capacitor. This contacting is very costly in large-scale production and can damage the oxide layer.
In European patent applications EP-A-1713103 and EP-A-1746613, a polymeric outer layer is generated by applying a dispersion comprising particles of a conductive polymer and a binder. Polymeric outer layers can be generated relatively easily using these methods. However, this method has the drawback that the ESR rises markedly under a thermal load, such as is produced for example during soldering of the capacitors. In addition, it is desirable to further reduce the ESR.
There was thus a need to improve the two methods described in EP-A-1713103 and EP-A-1746613 for producing solid electrolytic capacitors in such a way that the ESR decreases further and does not increase markedly even under increased thermal loading.